An electric power steering device is a device that generates steering assist force by the use of a motor according to a driver's steering torque. Recently, a demand for an electric power steering device for large-sized vehicles has rapidly increased. The large-sized vehicles require an increasing of steering assist force. Therefore, further large electric power has to be supplied to a motor. However, a battery as a main power source alone may not supply such large electric power sufficiently. Therefore, a configuration is suggested in which the battery is provided with an auxiliary power source of a secondary battery, only the battery is used in a normal state, and the battery and the auxiliary power source are connected in series to each other to supply electric power from both power sources when the larger electric power is required (for example, see Patent literature 1).